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Visualization of binding and uptake of oxidized low density lipoproteins by cultured mesangial cells
Background:
It is hypothesized that oxidative modification of low density lipoprotein (LDL) in glomeruli may play an important role in the progression of initial glomerular injury to glomerulosclerosis. Recent biochemical studies have shown that cultured rat mesangial cells (RMC) express the scavenger receptors for oxidized LDL (Ox-LDL) suggesting that mesangial cells participate in the development of glomerulosclerosis through intracellular lipid loading. Yet it is not clear whether cultured human mesangial cells (HMC) also have receptors to take up Ox-LDL for foam cell formation.
Experimental Design:
Colloidal gold or [125I]-labeled LDL was oxidized with copper ions. Binding experiments were performed by incubating the cultured human and rat mesangial cells at 4 degrees C with colloidal gold or [125I]-labeled Ox-LDL conjugates. The specificity of the [125I]-Ox-LDL binding was tested by competition experiments. Uptake and degradation studies were conducted by incubating the cells with labeled Ox-LDL at 37 degrees C.
Results:
When the cells were incubated with Ox-LDL-gold particles for 2 hours at 4 degrees C, gold particles associated with noncoated plasma membrane or coated pits were only found in RMC, but not in HMC. The binding of Ox-LDL-gold to RMC was prevented by an excess of unlabeled Ox-LDL, polyinosinic acid or fucoidin. When the cells were incubated with increasing concentrations of [125I]-Ox-LDL, the specific binding of [125I]-Ox-LDL increased in both cells. The specific binding of [125I]-Ox-LDL (10 micrograms/ml) was 23% of the total binding for HMC and 47% for RMC, respectively. After incubation for 4 hours at 37 degrees C with Ox-LDL-gold conjugates only RMC, in particular, phagocytic mesangial cells exhibited extensive internalization of gold particles developing into foam cells.
Conclusions:
The results indicate that HMC have a small number of specific receptors for Ox-LDL and therefore a scavenger receptor-mediated pathway for Ox-LDL was not visualized. In contrast, RMC, particularly phagocytic cells, express a large number of specific receptors for Ox-LDL generating foam cells.